When loads are placed on the surface of soft, saturated clay deposits, large settlements often result because of compression of the clay material. In saturated material, this settlement can take place only as pore water is expelled. If the permeability of the compressible soil is very low, this process takes place very slowly. Total settlements of several meters are common and often take years to occur. This time-dependent process is called consolidation. A process called sand drains and surcharging has been used in these cases since the 1920's (See D. E. Moran, U.S. Pat. No. 1,598,300).
In this process sand drains (columns of sand) are installed vertically on a regular area pattern through the soft layer to be treated. After the sand drains are installed, a sand or gravel drainage blanket one to three feet thick is placed over the drains to permit water to flow out of the drains. An earth embankment is placed over this drainage blanket. The thickness of the embankment or surcharge is normally calculated to produce loading roughly 10% greater than the anticipated final design load planned for the project.
The sand drains now provide free drainage paths within the clay mass. Without drains, drainage from any point within the clay must take place vertically, either to the surface, or downward to a permeable soil layer below, if such layer is present. With drains present, the drainage distance from any point within the clay is to the nearest drain. Drains are spaced so that drainage paths are much shortened, and consolidation occurs much more rapidly. The surcharge is left in place until the consolidation process is nearly complete (commonly about 90%). This creates a condition where the soil skeleton (or soil grains) is loaded to a level equal to or somewhat greater than the anticipated design load. The surcharge is then removed and the project proceeds. Since the soft soil skeleton has been precompressed to a load somewhat greater than the design load, no more settlement occurs.
In the late 1960's and early 1970's, wick drains were developed as an alternative to sand drains. Wick drains are not truly wicks, but are composite drains composed of an extruded flexible plastic core shaped to provide drainage channels when the core is wrapped in a special filter fabric. See, for example, U.S. Pat. No. 5,820,296. The filter fabric (geofabric or geotextile) acts as a filter, constructed with opening sizes which prevent the entrance of soil particles, but allow pore water to enter freely. The finished wick material or drain is strip or band-shaped, typically about ⅛ to ¼ inch thick, and approximately 4 inches wide. It is provided in rolls containing 800 to 1000 feet of drain. An example manufacturer is Nilex Corporation of Englewood, Colo. USA. Its product is sold under the trademark MEBRADRAIN.
More recently wick drains have been used to aid in the removal of contaminants from soil or aquifers (See, for example, U.S. Pat. No. 4,582,611). In one variation of this process, wick drains are inserted into the contaminated soil or aquifer, water is injected into one or more of the wick drains, and water with contaminates is removed from one or more wick drains.
Another recent development is the use of larger composite drains as a replacement for the sand or gravel drainage blanket. These drains are similar to wick drains but with much larger cross sectional area. They are placed to accept drainage out of the vertical drains and to provide horizontal drainage from under the surcharge. This “under drain system” is very efficient, and is usually cost-effective when compared with a sand or gravel layer.
In another variation, the surcharge may be replaced by a system that applies atmospheric pressure to the ground surface. To apply this method an impervious membrane is placed over the area to be consolidated. The edges of this membrane are placed into a trench and buried to provide an airtight seal around the perimeter of the membrane. A vacuum is then drawn from under the membrane. A system of horizontal drains, as just mentioned, is placed under the membrane and distributes the effects of the vacuum uniformly throughout the treated area. The maximum pressure that can be realized in practice is about 70% to 80% of atmospheric, and is equivalent to approximately a 15-foot high embankment.
Another application for vertical prefabricated drains in ground improvement is for liquefaction mitigation and remediation. One of the most destructive effects of earthquakes is their effect on deposits of saturated loose, fine sand or silty sand, causing a phenomenon known as liquefaction. When liquefaction occurs the soil mass loses all shear strength and behaves temporarily as a liquid. Such temporary loss of shear strength can have catastrophic effects on earthworks or structures founded on these deposits. Major landslides, lateral movement of bridge supports, settling or tilting of buildings, and failure of waterfront structures have all been observed in recent years, and efforts have been increasingly directed toward development of methods to prevent or reduce such damage.
When loose sand is subjected to repeated shear strain reversals, such as caused by an earthquake, the volume of the sand will decrease. If the sand is saturated and drainage out of the sand is prevented, it will be understood that since the volume of the sand is decreasing, the pressure of the water must increase. As the water pressure becomes greater the grain-to-grain contact pressure in the sand must become smaller and smaller. When this grain-to-grain contact pressure becomes zero, the entire sand mass will lose all shear strength and will act as a liquid. This phenomenon is known as liquefaction and can occur in loose, saturated sand deposits as a result of earthquakes, blasting, or other shocks.
Treatment of soil to improve liquefaction resistance has taken the form of densifying the soil, providing reinforcing elements within the soil, providing drainage, or some combination of these. Traditionally the most cost effective of these alternatives has been the use of stone or gravel columns to provide reinforcement and/or drainage. Such columns are spaced at intervals within the liquefiable soil. Although the stone or gravel column method has been used extensively in the past, recent research has called into question its effectiveness. For example, see “Drainage Capacity of Stone Columns or Gravel Drains for Mitigating Liquefaction,” Boulanger, R. W., Idriss, I. M., Stewart D. P., Hashish, Y, and Schmidt, B., 2nd Geotechnical Earthquake Engineering and Soil Dynamics Conference, Seattle, Vol. I, 678-690, 1997, and “Mechanical Behavior of Stone Columns Under Seismic Loading,” Goughnour, R. R. and Pestana, J. M., 2nd Int. Conf. On Ground Improvement Techniques, 7-9 October, 1998, Singapore.
One recently developed method of treating liquefiable soil for earthquake protection, comprises a plurality of substantially vertical prefabricated drains positioned at spaced intervals in the liquefiable soil and a reservoir, which is adapted for draining off water that is expelled from these composite drains (see U.S. Pat. No. 5,800,090). The object is to provide pore water pressure relief from a series of spaced locations within a liquefiable soil by providing an open drainage path, which operates as efficiently as possible-i.e. requires as little pressure as possible to move the required amount of water.
In the previous application where vertical drains were used for consolidation acceleration, drainage through the drains normally takes place over a period of several weeks, months, or even years. In this case, drainage must take place during strong shaking of the earthquake event, which is only a matter of seconds. The drains used in this application must provide flow capacity at least two orders of magnitude greater than normal wick drains.
One product that meets this requirement is the larger composite drains as mentioned above. This product is similar to wick drains but with a thickness of 1 to 1½ inches, and a width of 6 inches or more. Another recently developed product is corrugated plastic pipe. This product is perforated or slotted and can be wrapped in a geofabric. When used for liquefaction mitigation this product will have an inside diameter of from 2 to 10 or 12 inches.
Installation of vertical drains is accomplished by means of specialized equipment, consisting of a crane-mounted, vertical mast housing a special installation mandrel. The mandrel, containing the drain, is intruded by force directly into the ground from the bottom of the mast. After reaching the desired depth, the mandrel is withdrawn back into the mast, leaving the undamaged drain in place within the soil. For example, see U.S. Pat. No. 5,213,449. Sometimes vertical vibration is applied to the mandrel to aid in penetration. Typical spacing for wick drains is from three to ten feet. This well proven method of ground improvement has found extensive application where foundation materials are saturated and compressible, with moisture contents up to 100%. Such foundation materials include clays; soft, fine silts; organic deposits; and peat or “muck”. This method is very cost-effective and has virtually replaced the older sand drain method.
Installation of drains intended for liquefaction remediation (earthquake drains) is accomplished with similar equipment. The mandrel is larger to accommodate a larger drain cross sectional area. As with wick drains, vibration is often applied to the mandrel to assist in penetrating the soil. However, in this case, the primary purpose of vibration is to densify the soil, since liquefaction potential is also reduced as a result of soil densification. Commonly fins are added to the mandrel to improve transmission of vibration to the soil, thus enhancing the densification process. Densification of the soil is accomplished simultaneously with drain installation. Earthquake drains spacings normally vary from 2 to 6 or 7 feet.
U.S. Pat. No. 6,312,190 discloses a method and apparatus for enhancing the effectiveness of prefabricated composite vertical drains. This is accomplished by actively pumping water from the drain for some period of time. Temporarily pumping water from the drain will carry fine soil material out of the soil and into the drain. This suspended fine soil is pumped out of the drain and disposed of. Removal of fine soil material in the vicinity of the drain will increase the permeability of the soil near the drain, thus permanently enhancing the effectiveness of the drain.